INTRUSIVE IGNEOUS ROCKSChapter 4Polymerization of Silica• Each Si atom is surrounded by 4 oxygens.• Polymerization is the degree to which theoxygens are shared with other Si atoms.• Polymerization is the number of Si-O-Si bonds.– Olivine has none: isolated SiO4 groups– Pyroxene has one third: SiO3 chains– Mica has has 40% shared: Si4O10 sheets– Quartz and feldspar have 100% (Si,Al)O2framework.Polymerization of SilicaTetrahedronOlivinePyroxenePolymerization of SilicaMica (Sheet)Quartz (Framework)MAGMA•Magma is the term for any molten silicatematerial, whether below the surface or on top.•Volcanic rocks are erupted on the surface.– Volcanic rocks are fine-grained (<1 mm)• Intrusive igneous rocks crystallizefrom magma below the surface.– Intrusive igneous rocks are coarse-grained (> 1mm)–Pegmatites are very coarse grained (> 1cm)Intrusive Igneous Rocks• Composition• Mineralogy• Geologic Setting–Mantle– Oceanic Crust– Continental Crust• Igneous Fractionation– How the chemistry evolvesIgneous Rock Compositions• Rock compositions are described in weightpercents of oxides.• The principal variation in igneous rockcompositions is silica (SiO2) content.• The degree of polymerization of silicaincreases with silica content. (both incrystals and melt)Igneous Rock Compositions• Igneous rocks vary in composition from– ultramafic (~40 wt%) (peridotite)– mafic (48-55%) (gabbro)– intermediate (55-65%) (diorite)– silicic (65-75 wt %) (granite)• The mantle is peridotite;• The ocean basins gabbro;• The continents are granite.Igneous Rock Namesand Compositions• IntrusiveRock Name• Peridotite•Gabbro• Diorite•Granite• Compositions• Ultramafic•Mafic• Intermediate• Silicic (felsic)• VolcanicRock Name• (Komatiite)• Basalt•Andesite• RhyoliteIgneous Rock Mineralogy• Perodotite (Ultramafic)– Olivine ((Mg,Fe)2SiO4)– Pyroxene((Mg,Fe,Ca)SiO3)–Garnet (Mg3Al2Si3O12)– Spinel (MgAl2O4)• Gabbro (Mafic)– Feldspar(CaAl2Si2O8)– Pyroxene((Mg,Fe,Ca)SiO3)– Olivine((Mg,Fe)2SiO4)• Diorite (Intermediate)– Feldspar (CaAl2Si2O8)– Feldspar (NaAlSi3O8)– Pyroxene ((Mg,Fe,Ca)SiO3)–MicaK(Mg,Fe,)3(Si,Al)2O10(OH)2Igneous Rock Mineralogy• Granite (Silicic)–Quartz (SiO2)– Feldspar (NaAlSi3O8)– Feldspar (KAlSi3O8)– Mica (biotite)(K(Mg,Fe,)3(Si,Al)4O10(OH)2– Mica (muscovite)–(KAl2(Si,Al)4O10(OH)2Geologic Settings••Where?Where?•Mantle• Oceanic• Continental Mtns• Continental Mass• Compositions• Ultramafic•Mafic• Intermediate• Silicic (felsic)Igneous Fractionation:Peridotite-basalt-granite• Partial Melting– Rocks have a melting interval of several hundreddegrees C.– First melt is rich in silica, water and trace elements.• Fractional Crystallization– First-formed crystals low in silica settle to thebottom.Intrusive forms•Pluton: Any large discordant intrusion atdepth–Batholith large > 100 km2–Stock small < 100 km2•Dike Tabular non-conformable•Sill Tabular
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